scripts: fix dumping execution backtrace

Dumping the backtrace has been broken since changeset #652e56d6d35a:
scripts: execute each steps in a subshell

We can spawn sub-sub-shells in some cases.

The way the fault handler works is to dump the backtrace, but to avoid
printing it once for every sub-shell (which could get quite confusing),
it simply exits when it detects that it is being run in a sub-shell,
leaving to the top-level shell the work to dump the backtrace.

Because each step is executed in its own sub-shell, the variable arrays
that contain the step name, the source file and line number, are lost
when exiting the per-step sub-shell.

Hence, the backtrace is currently limited to printing only the top-level
main procedure of the shell.

Fix this thus:
- when dumping the bckatraces for the steps & the functions, remember
it was dumped, and only dump it if it was not already dumped
- at the top-level shell, print the hints

Also, rename the top-level step label.

Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 4193d6e6a17430a177fa88c287879c2c35e319f3)

 (C) 2007 Yann E. MORIN <yann.morin.1998@anciens.enib.fr>

 License: LGPL v2.1

 Shut off a stupid warning: preffered-stack-boundary must be between 4 ad 12,

 and 2 is not.

 diff -dur glibc-2.5.orig/sysdeps/i386/Makefile glibc-2.5/sysdeps/i386/Makefile

 --- glibc-2.5.orig/sysdeps/i386/Makefile	2005-03-06 01:18:16.000000000 +0100

 +++ glibc-2.5/sysdeps/i386/Makefile	2007-05-27 17:49:37.000000000 +0200

 @@ -36,7 +36,7 @@

  ifeq ($(subdir),csu)

  sysdep-CFLAGS += -mpreferred-stack-boundary=4

  else

 -sysdep-CFLAGS += -mpreferred-stack-boundary=2

 +sysdep-CFLAGS += -mpreferred-stack-boundary=4

  # Likewise, any function which calls user callbacks

  uses-callbacks += -mpreferred-stack-boundary=4

  # Likewise, any stack alignment tests